AMaSiS 2018 Workshop: Abstracts

Poster Modelling of the influence of secondary barriers on impulse response of CIGS solar cells

Konrad Wiśniewski, and Paweł Zabierowski

Warsaw University of Technology, Dept. of Physics

The response of thin-film CIGS solar cells, the most important part of which, the absorber, is a material from the Cu(In,Ga)Se2 family of compounds, for external electrical impulses, has not found a generally accepted explanation to this day. The multi-layered structure of CIGS cells is the main reason for disputes in this matter: when observing the change in the capacitance or the intensity of the current flowing through the test sample, one cannot be certain which part of the entire cell is responsible for these changes. As a standard, during the analysis of experimental data only point defects within the structure are considered, and their so-called "signatures" should appear in the dependencies studied. However, the barriers between the contacts of various materials constituting the cell or the grain boundaries are equally important - the CIGS material is polycrystalline.

In order to explain the origin of unexpected behaviour of experimental dependencies 4 mutually exclusive models were proposed and the discussion is still ongoing. In this work we simulate capacitance transients by solving a set of differential equations describing the response to a voltage pulse of a solar cell consisting of two junctions and one resistance representing the neutral region. We assumed that each part of a device can be described by its steady sate current-voltage characteristic. We discuss how this extended two-diode model helps to understand previously unexplained distinctive features of N1 DLTS and RDLTS signals of CIGS solar cells: (i) the sign of signals and their amplitudes, (ii) the difference in the position and widths of RDLTS and DLTS peaks, and (iii) the increase of an amplitude at higher temperatures. We show that the expression for junction capacitance needs to be reformulated since the equivalent circuit and therefore the susceptance of a sample must be calculated differently.

This work is a prelude to the creation of a spatial model of the solar cell which will enable to describe the impulse response of the system containing grain boundaries.

Acknowledgments: This work have been partially supported by Beethoven II project AlkaCIGS under contract No 2016/23/G/ST5/04268.